A distant retrograde orbit (DRO), as most commonly conceived, is a spacecraft orbit around a moon that is highly stable because of its interactions with two Lagrange points (L1 and L2) of the planet-moon system.
In more general terms, an object of negligible mass can be in a DRO around the smaller body of any two-body system, such as planet–Sun or exoplanet–star.
Using the example of a spacecraft in a DRO around a moon, the craft would orbit in the direction opposite to the direction in which the moon orbits the planet. The orbit is "distant" in the sense that it passes above the Lagrange points, rather than being near the moon. If we consider more and more distant orbits, the synodic period (the period between two moments when the craft passes between the planet and the moon) gets longer and approaches that of the moon going around the planet, so that the sidereal period (the time it takes for the craft to come back to a given constellation as viewed from the moon) can become much longer than the orbital period of the moon. An example with Europa has a sidereal period about eight times the orbital period of Europa,
DROs have been researched for several decades, but as of 2022, no spacecraft have used such an orbit for an actual flight.
The stability of a DRO is defined in mathematical terms as having very high Lyapunov stability, where an equilibrium orbit is "locally stable if all solutions which start near the point remain near that point for all time."
By 2014, a lunar DRO was the preferred alternative under consideration for the NASA-proposed Asteroid Redirect Mission (ARM). This orbit would have had a lunar orbital altitude of approximately 61,500 km (38,200 mi), a distance somewhat greater than the distance from the Moon to either of the Earth-Moon L1 or L2 Lagrangian points. NASA subsequently cancelled work on ARM in 2017 and never funded the build of flight hardware nor issued any space launch contracts.
A distant retrograde orbit was one of the proposed orbits around Europa for the Jupiter Icy Moons Orbiter—principally for its projected stability and low-energy transfer characteristics—but that mission concept was cancelled in 2005.
Two system requirements for the NASA Lunar Gateway mention[ when? ] the use of lunar DRO's. Requirement L2-GW-0029, Single Orbit Transfer, states "the Gateway shall be capable of performing a single round trip transfer to Distant Retrograde Orbit (DRO) and back within 11 months." Requirement L2-GW-0026, Propulsion System Capability, states "the Gateway shall provide a fuel capacity that would support performing a minimum of two round-trip uncrewed low-energy cislunar orbit transfers between a near-rectilinear halo orbit (NRHO) and a distant retrograde orbit (DRO) and orbit maintenance for a period of 15 years between refueling."
In celestial mechanics, the Lagrange points are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of the restricted three-body problem in which two bodies are very much more massive than the third.
The Interplanetary Transport Network (ITN) is a collection of gravitationally determined pathways through the Solar System that require very little energy for an object to follow. The ITN makes particular use of Lagrange points as locations where trajectories through space can be redirected using little or no energy. These points have the peculiar property of allowing objects to orbit around them, despite lacking an object to orbit. While it would use little energy, transport along the network would take a long time.
A quasi-satellite is an object in a specific type of co-orbital configuration with a planet where the object stays close to that planet over many orbital periods.
In astrodynamics and aerospace, a delta-v budget is an estimate of the total change in velocity (delta-v) required for a space mission. It is calculated as the sum of the delta-v required to perform each propulsive maneuver needed during the mission. As input to the Tsiolkovsky rocket equation, it determines how much propellant is required for a vehicle of given mass and propulsion system.
In orbital mechanics, a free-return trajectory is a trajectory of a spacecraft traveling away from a primary body where gravity due to a secondary body causes the spacecraft to return to the primary body without propulsion.
A halo orbit is a periodic, three-dimensional orbit near one of the L1, L2 or L3 Lagrange points in the three-body problem of orbital mechanics. Although a Lagrange point is just a point in empty space, its peculiar characteristic is that it can be orbited by a Lissajous orbit or a halo orbit. These can be thought of as resulting from an interaction between the gravitational pull of the two planetary bodies and the Coriolis and centrifugal force on a spacecraft. Halo orbits exist in any three-body system, e.g., a Sun–Earth–orbiting satellite system or an Earth–Moon–orbiting satellite system. Continuous "families" of both northern and southern halo orbits exist at each Lagrange point. Because halo orbits tend to be unstable, stationkeeping may be required to keep a satellite on the orbit.
65803 Didymos is a sub-kilometer asteroid and synchronous binary system that is classified as a potentially hazardous asteroid and near-Earth object of the Apollo group. The asteroid was discovered in 1996 by the Spacewatch survey at Kitt Peak, and its small 160-metre minor-planet moon, named Dimorphos, was discovered in 2003. Due to its binary nature, the asteroid was then named Didymos, the Greek word for 'twin'.
Asteroid capture is an orbital insertion of an asteroid around a larger planetary body. When asteroids, small rocky bodies in space, are captured, they become natural satellites. All asteroids entering Earth's orbit or atmosphere so far have been natural phenomena; however, U.S. engineers have been working on methods for telerobotic spacecraft to retrieve asteroids using chemical or electrical propulsion. These two types of asteroid capture can be categorized as natural and artificial.
A subsatellite, also known as a submoon or moonmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet.
The Asteroid Redirect Mission (ARM), also known as the Asteroid Retrieval and Utilization (ARU) mission and the Asteroid Initiative, was a space mission proposed by NASA in 2013. The Asteroid Retrieval Robotic Mission (ARRM) spacecraft would rendezvous with a large near-Earth asteroid and use robotic arms with anchoring grippers to retrieve a 4-meter boulder from the asteroid.
Artemis 2, is the second scheduled mission of NASA's Artemis program, and the first scheduled crewed mission of NASA's Orion spacecraft, currently planned to be launched by the Space Launch System (SLS) in May 2024. The crewed Orion spacecraft will perform a lunar flyby test and return to Earth. This is planned to be the first crewed spacecraft to travel beyond low Earth orbit since Apollo 17 in 1972. Formerly known as Exploration Mission-2 (EM-2), the mission was renamed after the introduction of the Artemis program. Originally, the crewed mission was intended to collect samples from a captured asteroid in lunar orbit by the now canceled robotic Asteroid Redirect Mission.
A flyby is a spaceflight operation in which a spacecraft passes in proximity to another body, usually a target of its space exploration mission and/or a source of a gravity assist to impel it towards another target. Spacecraft which are specifically designed for this purpose are known as flyby spacecraft, although the term has also been used in regard to asteroid flybys of Earth for example. Important parameters are the time and distance of closest approach.
The Lunar Gateway, or simply Gateway, is a planned small space station in lunar orbit intended to serve as a solar-powered communication hub, science laboratory, short-term habitation module for government-agency astronauts, as well as a holding area for rovers and other robots. It is a multinational collaborative project involving four of the International Space Station partner agencies: NASA, European Space Agency (ESA), Japan Aerospace Exploration Agency (JAXA), and Canadian Space Agency (CSA). It is planned to be both the first space station in deep space and the first space station to orbit about the Moon.
EQUULEUS is a nanosatellite of the 6U CubeSat format that will measure the distribution of plasma that surrounds the Earth (plasmasphere) to help scientists understand the radiation environment in that region. It will also demonstrate low-thrust trajectory control techniques, such as multiple lunar flybys, within the Earth-Moon region using water steam as propellant. The spacecraft was designed and developed jointly by the Japan Aerospace Exploration Agency (JAXA) and the University of Tokyo.
Dimorphos is a small asteroid satellite that was discovered in 2003. It is the minor-planet moon of a synchronous binary system with 65803 Didymos as the primary asteroid. After being provisionally designated as S/2003 (65803) 1 with informal nicknames such as "Didymos B" and "Didymoon", the Working Group Small Body Nomenclature (WGSBN) of the International Astronomical Union gave the satellite its official name on 23 June 2020. At a diameter of 170 metres (560 ft), it is one of the smallest astronomical objects that has been given a permanent name.
The Power and Propulsion Element (PPE) previously known as the Asteroid Redirect Vehicle propulsion system is a planned solar electric ion propulsion module being developed by Maxar Technologies for NASA. It is one of the major components of the Gateway. The PPE will allow access to the entire lunar surface and a wide range of lunar orbits and double as a space tug for visiting craft.